"[If it hit Earth] it makes the bang of a very large nuclear weapon – a very large one."

How big?

"It would have hit with over 30 times the energy of the atomic blast at Hiroshima," says Swinburne University astronomer Associate Professor Alan Duffy.

"It's a city-killer asteroid. But because it's so small, it's incredibly hard to see until right at the last minute.

"It's threading tightly between the lunar orbit. Definitely too close for comfort."

Asteroids this size tend to pass by once every decade.

Three other asteroids also raced past the Earth on Thursday, but none were as close or as large as 2019 OK. Because it was only just spotted, its existence has not yet been widely reported.

The asteroid was picked up by separate astronomy teams based in Brazil and the US over the past few days.

NASA's Jet Propulsion Laboratory confirmed the discovery. The asteroid passed Earth just 73,000 kilometres away, and was sized between 57 and 130 metres in diameter, according to the lab's data.

"This is one of the closest approaches to Earth by an asteroid that we know of. And it’s a pretty large one," says Professor Brown.

By comparison, the rock that killed the dinosaurs was about 16 kilometres across, said Professor Gretchen Benedix, a planetary science researcher at Curtin University.

"It’s not totally out to lunch, these things happen. It’s more rare they happen within a lunar distance," she said.

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"If that were to hit the Earth, that would be bad. Something 100 metres across would leave a noticeable hole on the planet."

The Chelyabinsk meteor, which exploded over Russia in 2013, was only about 20 metres in diameter.

Astronomers typically try to pick up asteroids long before they pass by Earth.

But this one was particularly difficult to see because it was coming toward the Earth from the direction of the sun, Professor Brown said.

"It was faint, it was close to the sun. It’s been getting closer to us, getting brighter and brighter, and finally some smaller telescopes have picked it up. Literally, right about now, it’s about 70,000 kilometres from Earth," he said.

A person armed with a pair of binoculars and looking at the right spot in the night sky may even have been able to spot it, he said.

Several dozen smaller asteroids in the six-to-12 metre range fly past Earth at a distance closer than the moon every year, according to NASA.

But such a large rock passing so close is unusual.

"These events are rare. But we know, sooner or later, there's going to be one with our name on it," says Professor Duffy.

Australian National University astronomer Dr Brad Tucker said it may be that many rocks of similar size whizz by close to the Earth – we just don't detect them.